The dual-X has bigger fans. Probably quieter and slightly cooler. BUT, jrgong's right, the IceQ X is cheaper and has a better cooler than both.
BUT jrgong's wrong, HD 7850 is perfect for 900p. Especially if paired with a recent i5 (no chance for a CPU bottleneck). You're looking at 45+fps in everything, including antialiasing. I'd get one if it were more than a 20% jump from my current card.
Also, CrossfireX is pairing two cards to work in tandem. Nearly doubles performance but has a few drawbacks.